The invention relates to a method of heating glass, the method comprising conveying glass through a tempering furnace so that the glass is heated from above and below, an upper surface of the glass being heated by hot air jets formed by sucking air from inside the furnace and pressurizing the hot air and recycling it back to the upper surface of the glass.
The invention further relates to an apparatus for heating glass, the apparatus comprising a tempering furnace comprising horizontal rolls arranged to carry the glass and to form a conveyor thereof, an upper side return pipe for sucking air from inside the tempering furnace, a pressurization unit for pressurizing the air sucked from inside the tempering furnace, and means for blowing the pressurized air back to an upper surface of the glass.
A problem involved in heating glass using an oscillating roll furnace is that the edges of the glass tend to curve upwards at the beginning of the heating. This is due to the large heat flow received by the lower surface of the glass at the beginning of a heating cycle, caused by the ceramic rolls used in the furnace, compared to the heat flow received by the upper surface of the glass. As a result, the edges of the glass curve upwards while the middle area of the glass tends to suffer from optical errors; in addition, the glass heats up unevenly. When heating selectivity glass, the situation is particularly difficult since selectivity glass reflects thermal radiation extremely strongly. Glass with a selective surface is usually heated with the selective surface upwards, so it is difficult to heat the upper surface of the glass in particular, compared to heating the lower surface thereof. In such a case, the heating times for selective glass are, of course, considerably longer than those for common clear glass; therefore, when heating selective glass, the capacity of a furnace is typically quite low.
WO publication 97/44283 discloses a solution wherein the underneath surfaces of glass are cooled at the beginning of a heating cycle. Furthermore, heat transfer on the underside is enhanced towards the end of the heating cycle by blowing hot air directly to the lower surface of the glass. The solution is extremely effective and well-working but, when heating selective glass in particular, it would be advantageous if the heating time could be shortened.
WO publication 01/32570 discloses a solution wherein glass is heated such that air is blown to the upper surface of the glass in order to heat the glass. The blown air is sucked from inside a tempering furnace, so it is already hot. The sucked hot air is pressurized and recycled back by blowing it through a pipe system to the vicinity of the upper surface of the glass, substantially perpendicularly to the upper surface thereof. The lower surface of the glass is also heated in a similar manner by hot air jets that are produced by taking air from inside the furnace and recycling the hot air back after pressurization. The solution works extremely well when heating selective glass. The solution enables the heating time for selective glass to be reasonably short. However, the structure of hot air pressurization devices is rather complex and therefore quite expensive. Furthermore, thermal expansions caused by the blowing of hot air set strict requirements for the structure of blowpipe systems, which makes such blowpipe systems quite complex and expensive.